When used in conjunction with the imaging capabilities of an electron microscope, cathodoluminescence is a powerful tool for analysis in a variety of different technical areas. By way of example, cathodoluminescence provides an especially powerful analysis tool in the study of semiconductors. The energy levels in semiconductor materials and insulators are affected by the concentrations of impurities, carriers and electrically active defects. Analysis of cathodoluminescence spectra enables the experimentalist to characterize and determine the effects of these and other phenomena on the cathodoluminescence emission. See for example, Cathodoluminescence Scanning Electron Microscopy of Semiconductors by Yacobi and Holt, Journal of Applied Physics, Volume 59 No. 4 February 15, 1986 pages R1 to R24.
Cathodoluminescence from biological samples originates from the material itself or from fluorescent dyes introduced for selective staining. Use of cathodoluminescence provides additional information to the electron microscopist and has wide application to plant and animal tissues. See for example, Scanning Electron Microscopy of Biological Material Using Cathodoluminescence by E. M. Horl, MICRON, 1972 3:540-544. Cathodoluminescence has a very wide range of applications in geology and minerology. The image can be affected by features such as distortions or cracks in the crystal lattice, omission defects in the crystal lattice, local crystal inhomogeneities and the presence of impurities within the lattice. For example, diamonds can be characterized from the cathodoluminescence spectra they emit and it is possible to differentiate between natural and synthetic diamonds. Cathodoluminescence also has application in the study of phosphors. Cathodoluminescence of phosphorescent materials provides valuable information relating to the efficiency of the phosphor and its microstructure.
The present invention may be used advantageously in a scanning electron microscope which generates a beam of electrons which is focused to a small spot on the sample surface. The beam is swept across an area of the sample in a raster pattern. In a conventional mode of operation, the image of the sample surface is formed by detecting the emission of secondary electrons from the sample and electronically correlating the secondary electron signal with the position of the beam. Other signals, carrying different information about the sample, can also be used for image formation. For example, back scattered electrons can be collected with a special detector to produce an image which is sensitive to variations in the material composition of the surface.
The electron beam induces a variety of excitations that result in emitted radiation. In particular, electron-hole pairs are produced in the interaction volume of the electron beam with the sample. The carriers generated by this process exist for a characteristic period of time during which they undergo intraband relaxation and diffusion/drift before recombining. Recombination can be accompanied by the emission of a photon, which is particularly likely if the sample is for example, a direct gap semiconductor. In this particular case, the photon will have an energy level close to the local band gap energy of the semiconductor. The light generated by this process, cathodoluminescence, can be used to study the local properties of the material and the results can be correlated with information from other scanning electron microscope imaging techniques.
The present invention also finds particularly advantageous application in a scanning transmission electron microscope where the emission collection system is usually constrained to a very small size. See for example, Scanning Transmission Electron Microscopy Techniques For Simultaneous Electronic Analysis and Observation of Defects In Semiconductors, by Petroff, Lang, Strudel and Logan, Scanning Electron Microscopy /1978/Vol. I, pgs. 325 to 332.
The use of cathodoluminescence light collection in electron microscopes as well as the use and analysis of spectra and images derived from cathodoluminescence light collection are not, per se, new. One example of relevant prior art known to the applicants consists of the use of a parabolic or ellipsoidal-shaped collection mirror. In the ellipsoid mirror case, one focus is centered on the area of interest of the sample while the other focus lies outside the vacuum window of the microscope column. The second focus can be aligned at the entrance to various optical components such as light guides, fiber optic bundles or a spectrometer. The mirror is usually positioned above the sample and separated therefrom by a one to three millimeter clearance. Adjustment of the mirror over the sample is usually achieved by a mechanism inside the support housing of the mirror using micrometer drives in three mutually perpendicular directions.
Unfortunately, the parabolic or ellipsoid mirror cathodoluminescence collection system of the prior art, presents a number of significant disadvantages. By way of example, one such disadvantage is the size and the clearance requirement of the mirror which make this type of system incompatible with a large number of electron microscopes. Another such disadvantage is that because the mirror is normally positioned over the sample, it is not possible to provide other forms of simultaneous electrical stimulation (such as with a probe) and signal collection (e.g., X-ray detection) thereby rendering the prior art less convenient to use and potentially less useful for analysis. Still another disadvantage of the prior art mirror collection technique of the prior art is that the mirror must be periodically removed from the scanning electron microscope and cleaned to prevent a significant reduction in light collection capability, and of course, in such required periodic cleaning is a risk of mirror surface damage which detracts from the inherent advantage of cathodoluminescence analysis.
There has therefore been a long felt need for a cathodoluminescence light collection system for use in electron microscopes which obviates the prior art requirement for large collection surfaces such as a collection mirror, while still providing the numerous advantages of cathodoluminescence analysis.